This invention relates to warewashers and more particularly to a method for achieving satisfactory washing results where the available water supply is not hot enough. The invention has particular application in domestic dishwashing machines.
It is common in the United States for manufacturers to specify that the water supply temperature for a warewasher must be at least 140.degree. F. at the machine. In some domestic applications the central hot water tank is set above 140.degree. F., but is located a long distance from the dishwashing machine, so that the water loses heat during transmission from the hot water tank. In other cases the hot water heater is set below 140.degree. F. The latter is understandable since, for almost every other domestic application, water at 120.degree. F. is satisfactory.
In the past, if washing results in a domestic dishwashing machine were not satisfactory, one could simply increase the temperature of the hot water heater. With today's increasing energy costs and greater emphasis upon energy conservation, such a remedy is no longer expedient. In fact, even where there are no substantial heat losses, it is increasingly difficult to justify raising the hot water supply temperature for the entire household merely to meet the needs of the dishwashing machine.
Nevertheless, for satisfactory results it is currently necessary to have a temperature within the machine of 130.degree. F. -140.degree. F. When water temperatures fall much below that range, certain foodstuffs, for example fats, may not be properly removed. Common dishwashing detergents also quickly lose their effectiveness as temperatures fall. At temperatures below 105.degree. F. -110.degree. F. the detergents and food proteins cause sudsing which seriously impairs the effectiveness of the recirculating and spraying system within the dishwashing machine.
Of course, there are low temperature detergent formulations which are reasonably effective at reduced temperatures. However, such detergents are expensive and are not widely available at present. Therefore, the need which now exists is for a domestic dishwashing machine which is at once compatible with current domestic dishwashing practices and which will at the same time provide satisfactory washability with water supplied at reduced temperatures.
The obvious answer is to add heat to the water within or just before reaching the dishwashing machine to bring the water temperature up to an acceptable level. Dishwashing machines with a sump heater have in fact been known and available for many years. For example, U.S. Pat. No. 3,707,156, assigned to the assignee of the present invention, shows a domestic dishwashing machine in which a first water heating element is energized during washing periods to maintain the wash water temperature against ordinary losses to the outside, and a second heating element may be energized during a final rinse period, at the user's option, to raise the water temperature to a higher level for more effective sanitizing of the ware prior to drying.
U.S. Pat. Nos. 4,159,211, 4,070,204 and 3,440,399 provide examples of domestic dishwashing machines designed to compensate for an inadequate water supply temperature. These machines have thermostatically controlled heaters for adding heat as long as the water temperature is below a predetermined level. Further, the '211 and '399 devices interrupt operation of the timer motor during certain portions of the cycle to prevent the cycle from progressing further until the predetermined temperature is reached. Unfortunately, this can result in unacceptably prolonged washing cycles.
A domestic dishwashing machine, as a practical matter, cannot be designed to draw "unlimited" amounts of power. This is not because such a machine might be uneconomical to operate, but because in many homes the maximum electrical service available is only 15 or 20 amps. Therefore, if the recirculating motor draws 600-700 watts, the water heater must be limited to about 700 watts if it is to be operated simultaneously with the motor. Subtracting heat losses to the ambient during recirculation of the water within the machine, the net temperature rise during 700 watt heating is very slow. Further, the overall power consumption during this time is nearly twice as great (due to the motor), so that a prolonged heating cycle can actually become very expensive. Even worse, the suspended food soil debris can be pulverized and disintegrated to such an extent that large quantities of it will no longer be stopped by the fine filter in the recirculating system. This can seriously increase the amount of food soil debris which is subsequently redeposited onto the food ware items within the dishwashing machine.
A need thus remains for a domestic dishwashing machine which will provide excellent washability with reduced supply water temperatures, in acceptable cycle time periods, and without excessive energy consumption. One solution, for example, might be to spray the food ware items with the supply water while it is being admitted, as is commonly done in many machines, to pre-rinse them by removing the loose soil therefrom, and to warm them with whatever heat is available from the supply water. Then, to prepare for a wash cycle following this pre-rinse, the operation of the pump could be suspended and increased heat turned on to warm the water to approximately 150.degree. F. The pump could then be briefly operated to transfer this heat to the food ware items, following which the water would be drained and the succeeding wash cycle commenced. Unfortunately, although this might seem a logical way to go, it was discovered that the washability results are actually worse. Spotting, filming, and baked-on soil redeposition were unacceptable. In fact, additional final rinses had to be added to get adequate spotting and filming results, and even these were not satisfactory under certain field conditions. The above-noted needs for a domestic dishwashing machine, therefore, still remain.